In hydrocarbon producing wells, it is desirable to remove accumulated solid particles, sediment, and/or injection fluids, such as fracturing acids, sands and drilling fluids from the well bore to avoid restriction of the flow of hydrocarbons caused by such materials. When removing such materials it is important to keep formation damage to a minimum to ensure maximum production. This is particularly important when attempting to clean out low and under pressure reservoirs which are more susceptible to formation damage.
The drilling of low or under pressure reservoirs is quickly becoming more prevalent as conventional (i.e. normal pressure) sources of oil and gas become depleted. Currently in the United States, over 26% of the total gas production comes from low permeability or tight reservoirs, shallow gas wells, coal bed methane and shale gas. However, recovery of oil and gas from these low or under pressure reservoirs is difficult due to drilling damage, well stimulation damage and well completion damage. Such damage can make the difference between a commercial well or an abandoned well.
Conventional clean out methods and corresponding apparatus involve the pumping of high pressure air or fluids down a well bore. High pressure delivery of clean out fluid is necessary to lift undesirable material such as fracturing sand, drill cuttings, formation shales, stimulation acid, cement and the like to the surface. Unfortunately, the lifting pressures used will often exceed the formation pressure thereby forcing the accumulated materials into the production zones. This is particularly prevalent when dealing with the clean out of a low or under pressure reservoir. Zones which have the most porosity and permeability produce the most and damage the easiest.
Open hole completions can pose problems when performing conventional clean out operations because well bore material such as shale can collapse the well bore, cave it in, or fill up the well bore past the productive zones. Formation water from a zone can also shut-off or restrict flow from the productive zones.
As well, certain swelling clays such as bentonite can be present in producing formations. If water contacts these clays during conventional clean out operations, these clays can swell and permanently damage the producing formations.
Conventional clean out operations can also cause considerable damage to coal bed methane wells, which most often are completed open hole and are usually very low pressure. Coal is much softer than rock and it can be pulverized during drilling or clean out operations. The small particles of coal which result from this pulverization process are forced into the fractures in the coal seams by the current clean out and well drilling technology. This greatly reduces the ability of the methane gas to flow freely from the fractures in the coal and into the open hole well bore.
The present invention addresses some of the problems associated with conventional clean out procedures.